How do you know that no two snowflakes are alike? You can't count them anyway.
Sung Hee Kim, 12, Cincinnati, Ohio
There's no doubt about it. You can't keep track of every flake that falls from the sky. But amazingly, it takes billions and billions of molecules of water to create one snowflake! And those molecules crystallize (attach to each other to form solid matter) in an almost infinite number of ways. "The probability that you'll get two snowflakes exactly alike is close to zero," says Ken Libbrecht, a physicist and expert in ice crystals at the California Institute of Technology.
A snowflake begins its life in the clouds (water vapor and ice particles suspended in Earth's atmosphere) when air temperatures plunge below -water's freezing point (32 [degrees] F or 0 [degrees] C). Super-cooled water molecules don't freeze. They remain free-floating until they crystallize on a solid substance--like a dust particle. When water molecules freeze on dust, the icy speck floats around and collects more water molecules, causing the crystals to grow. And when it gets heavy enough, a snowflake tumbles down to Earth.
Snowflakes are usually hexagonal (six-sided) and symmetrical (same on all sides). This shape helps water molecules fit together to form solids. But the intricate snowflake pattern is dependent on temperature. "The way a snow crystal grows is very temperature-sensitive," says Libbrecht. Each snowflake forms at different altitudes and air temperatures; each travels miles and miles and endures different temperature changes on its journey to Earth. As it blows in one direction, the crystals will grow in that direction. "Then, when the snowflake moves through air at another temperature, suddenly all six arms grow a different way," says Libbrecht. By the time the snowflake hits the ground, "its complicated history is reflected in that one flake."
Libbrecht compares snowflakes to people: Each one has its own journey, goes to different places at different times, and encounters different things. "Hey, not even identical twins are exactly alike," adds Libbrecht.
For more on snow crystals, check out: www.snowcrystals.net